L

10 20 30

bed height (cm)

Fig. 7.9. Moisture profiles during growth of Aspergillus niger on wheat bran in a 35-cm-high packed-bed bioreactor (Gowthaman et al. 1993b; Ghildyal et al. 1994). (a) Temporal moisture profiles at different axial positions, for various different air flow rates. Key: bed heights of (-) 3.3 cm (---) 17 cm (■ ■ ■) 28 cm. (b) Axial moisture profiles, at different times, for various different air flow rates. Key: Times of (-) 16 h (---) 24 h (■ ■ ■) 40 h.

Adapted from a table presented by Gowthaman et al. (1993b) with kind permission from Elsevier

• At 3.3 cm height, the water content of the bed increased over time or stayed constant when the air flow rate was 20 L min-1 or below, but decreased at 25 L min-1, although even at 25 L min-1 this lower region did not dry out as quickly as the upper regions of the bed.

• The mid height of the bed (17 cm height) tended to dry out at all air flow rates.

• The water content of the bed at the upper position (28 cm height) increased during the fermentation for an air flow rate of 5 L min-1 but decreased with time for air flow rates of 15 L min-1 and above.

Inspecting the same results, but looking at the axial temperature gradients as a function of the air flow rate and how the axial temperature gradients varied over time, the key observations are (Fig. 7.9(b)):

• at 5 L min-1 the bed was driest at 17 cm, with the 3-cm and 280-cm heights remaining near and even exceeding the original water content of 51% (w/w). This observation holds for 16, 24, and 40 h.

• at 15 L min-1 the bed remained wet at 3 cm height, in fact, by 40 h it was significantly wetter than the original value, but became dry at 17-cm and 28-cm heights, with these two upper heights being reasonably close in water content, with values around 25-40 %(w/w)

• at 20 L min-1 the pattern was similar to that obtained for 15 L min-1 except that the water content at the 3-cm bed height remained close to the original value throughout.

• at 25 L min-1 all regions of the bed dried. By 16 h the water content had fallen to around 40% (w/w) at all bed heights. At 24 and 40 h the 3-cm height still had a water content around 40% w/w, but at both the upper bed heights the water content had fallen to around 22-26% (w/w).

These results suggest that drying patterns can be quite complex. Chapter 25 presents a model that can be used to explore these patterns.

Weber et al. (2002) monitored the off-gas relative humidities during packed-bed fermentations with two different fungi (Fig. 7.7(b)). At the time of the peak heat generation rate, the off-gas relative humidity fell to values around 90%. This could imply that either the transfer of water from the particle to the air is limiting or simply that the bed is drying out. It would be necessary to monitor the water activity of the bed contents in order to distinguish between these two possibilities.

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